The present invention relates to magnetic resonance imaging (MRI) and in particular to a method of manufacturing local coils used in MRI imaging.
Magnetic resonance imaging (MRI) reconstructs images, for example of the human body, by detecting a nuclear magnetic resonance (NMR) signal emitted from processing hydrogen protons, principally in watery tissue of the body. The NMR signal is faint and thus it is known in the art to place electrical coils on the patient in close proximity to the imaged tissue to improve the signal-to-noise ratio of the detected NMR signal.
A wide variety of different RF local coil designs are known in the art including single loop coils, paired loop coils (Helmholtz), quadrature coils, xe2x80x9cbird cagexe2x80x9d coils and phased array coils. Coils of these designs and others are shown generally in U.S. Pat. Nos. 4,734,647; 5,136,244; 5,166,618; 5,277,183; 5,370,118; 5,256,791 5,619,996; and 6,438,402, all assigned to the assignee of the present invention and hereby incorporated by reference.
The signal-to-noise ratio of the detected NMR signal may also be improved by reducing noise in the detected signal. To this end, local coils are usually tuned to a resonant frequency close to the expected Larmor frequency of the hydrogen protons whose resonance is being detected. For a 1.5 Tesla magnetic field in an MRI device, this frequency will be approximately 64 megahertz.
The tuning creates a sensitivity in the coil at the Larmor frequency of the expected NMR signal and a desensitivity to external noise sources at other frequencies. The sharpness of this frequency discrimination is a function of the electrical Q of the local coil which in turn is a function of the resistance of the coil conductors at the relevant NMR frequency.
Low resistance in the conductors of a local coil may be obtained by using conductors having large cross sections such as copper tubing. These conductors may be bent into the desired shape of the coil and tuned with series capacitances to the desired resonant frequency.
It can be difficult to manufacture coils using such conductors and the local coil that is produced is relatively rigid and may not readily conform to the patient. Accordingly, it is known in the art to construct local coils using printed circuit board material where the coils are precisely etched out of metal foil supported on an insulating substrate or formed using well-known additive techniques. The substrate may be flexible allowing the formed coils to better conform to the patient.
One disadvantage to the use of printed circuit board techniques is that the resistance of metal foil conductors is generally greater than the resistance of wire or tubular conductors. This higher resistance decreases the Q of the resultant local coil.
The present inventors have found a way to significantly increase the Q of local coils manufactured of printed circuit board materials by forming duplicate patterns in metal foil on both sides of the insulating substrate. The duplicate patterns may, but need not be, joined with conductors. At high frequencies, the duplicate patterns share current flow decreasing the intrinsic resistance of the local coil. The proximity of the two patterns is also believed to promote more uniform current distribution in the metal foil decreasing the effective resistance of the foil. Alternatively, for a given desired Q, this technique allows thinner and more flexible metal foil to be used. Importantly, the inventors have determined that two separated layers of metal foil, of a given thickness, provide benefits beyond those obtained with a single layer of foil of twice the thickness. Further, the invention allows coil designs normally requiring foil layers thicker than is available in standard printed circuit boards, for example four ounce copper, to be realized using standard circuit board materials in which the copper foil is normally limited to two ounces or less.
Specifically, the present invention provides an MRI local coil in which a first conductive pattern is formed on a front surface of an insulating sheet to provide a resonant antenna sensitive to NMR signals. A second conductive pattern is formed on a rear surface of the insulating sheet in alignment with the first conductive pattern and, signal terminals are attached to the first conductive pattern to conduct an NMR signal to an MRI machine for processing.
Thus, it is one object of the invention to provide a method of significantly increasing the electrical Q of an MRI coil constructed using standard printed circuit materials and techniques.
The first and second conductive patterns may be substantially identical.
Another object of the invention is to provide a technique that may make use of existing patterns used for printed circuit board coils simply by duplicating the patterns on the rear surface of the printed circuit board.
The insulating sheet may be a flexible material.
Thus, it is another object of the invention to provide an improved tradeoff between improved flexibility of thin metal foil used in flexible local coils and high electrical Q.
The first and second conductive patterns may be electrically connected by direct current.
Thus, it is another object of the invention to allow improved control of current flow in the first and second patterns by defining points of common voltage.
The electrical connections between the first and second conductors may be via plate-through holes.
Thus, it is another object of the invention to permit an electrical interconnection between the patterns using standard printed circuit board techniques.
The first conductive pattern may include series capacitances to tune it into resonance and, the second conductive pattern may have corresponding series capacitances.
Thus, it is another object of the invention to allow distribution of the necessary tuning capacitances on both the first and second conductive patterns to employ lower value capacitances with higher electrical Q values.
The first and second conductive patterns may be loops.
Thus, it is an object of the invention to allow the manufacturer of a building block of many local coils using this technique.
The insulating sheet may be in the form of a cylindrical tube.
Thus, it is another object of the invention to allow the construction of coils having non-planar topologies.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.